1. Field of the Invention
The invention relates to a single-layer or multi-layer paper sack, box-shaped when filled, which is closed at each of its two extremities by a simple singly or doubly folded wrapped end.
2. Description of the Prior Art
Prefabricated sacks, i.e. sacks already closed at both extremities and provided with a self-closing valve for filling, are generally designated valve sacks.
Such sacks have found wide application for packaging pulverous or granular materials, because in this way even sensitive material can be filled into self-closing prefabricated sacks without the need for a further closing operation after the filling operation.
Prior valve sacks are illustrated in FIGS. 1 to 6.
FIGS. 1 to 3 show a valve sack in which each of the two open flat-lying ends of the tube blank is covered by folded paper strips 4, 6 and sewn to the ends by the thread seam 5. FIG. 1 shows specifically the front elevation of this sewn valve sack, in which the back side of the sack is shown at 1 and the front side of the sack at 2. The sides are folded in the unfilled sack at 3a and 3b. FIG. 2 shows specifically the folded strip projecting in the open tube blank before the completion of the sack, which is tucked in before the application of the folded strip 6, as indicated by an arrow in FIG. 2. This produces the valve 8 (FIG. 1) with the interior filling orifice 9b after the sack is sewn. FIG. 3 shows this sewn sack in the filled state, in which the lateral folds 3a, 3b, which still lie virtually flat in FIG. 1, have now been opened by the contents to form the narrow side of the sack, which is virtually box-shaped in the filled state. Since the valve 7, 8 is folded in directly from the paper tube, a slight shortening 1 (FIG. 3) in the region of the valve occurs in the filled sack. Here, the closure strip 6 hangs freely and the sack extremity is not positively sewn, but is nevertheless closed by the tucked-in valve tube.
The sewn valve sack illustrated in FIGS. 1-3 has the advantage of extremely simply shaped sack extremities on both side. However, it has the disadvantage that high stresses occur in the filled state due to the large number of holes in the two thread seams 5, and also the sack is not powder-tight.
In contrast to the above, a sack of more complicated shape is illustrated in FIGS. 4-6. This sack is distinguished from the sewn valve sack by a cross end shaped at the two extremities of the initially open paper tube. A valve tube, wound from a paper sheet and glued is also let into the one of the two cross ends, and forms the filling valve for this valve sack. FIG. 6 shows specifically the still open valve sheet, which has been pre-folded in the two lines 11a, 11b and the line 12 before it is folded round initially in the top line 11a then in the bottom line 11b, so that a tube is produced. The shorter strip 4 is preglued so that the strip 14 is stuck locally to the strip 10 after folding in the two lines 11a, 11b. This valve tube is positioned in the end according to FIG. 4 so that the strips 10 with 14 in the shape illustrated form the valve bottom and the region 13 with the thumb cutout 13a form the valve top. As FIG. 4 further shows, the valve tube is inserted into the end and stuck before the strips 16b and 16a are folded and likewise stuck. The valve projects initially by its corners 17 and 18 beond the sides of the end triangle. However, after the valve sack is filled, the valve is preferably folded in or folded along the pre-creased line 12 and additionally closed with the stuck strip 19a of the end cover sheet 19 (FIG. 5) and sealed.
The valve sack according to FIGS. 4-6 apart from the tube blank 15, which in this case, in contradistinction to the sewn valve sack according to FIGS. 1-3, exhibits no lateral folds, is produced from the valve sheet according to FIG. 6 and the end cover sheet 19.
This valve sack, which is provided with a cross end at its two extremities, is more suitable for packaging sensitive materials than the sewn valve sack according to FIGS. 1-3, since it has better powder tightness. The cross bottom sack, however, has inwardly folded corner parts, known as "negative corners". These corners cannot be provided with glue and adequately sealed. Thus, open channels remain in the region of the cross bottom and leakage can occur; complete powder tightness can be achieved only with a further cover sheet according to FIG. 5. In comparison to a sewn valve sack, this valve sack according to FIGS. 4-6 is however considerably more expensive to produce than the sewn valve sack due to the complicated shape of the cross end.